The present invention is directed generally to controlling the transmission of information in communication systems. More particularly, the invention relates to controlling optical signal characteristics during transmission in information network, transmission and communication systems and controllers for use therein.
The continued development of digital technology has provided electronic access to vast amounts of information. The increased access to information has fueled an increasing desire to quickly obtain and process the information. This desire has, in turn, placed ever increasing demands for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems linking the processing equipment (i.e., telephone lines, cable television (CATV) systems, local, wide and metropolitan area networks (LAN, WAN, and MAN)).
In response to these demands, telecommunications companies have turned to optical communication systems to provide substantially larger information transmission capacities than corresponding electrical systems. Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one signal in a single wavelength per waveguide, i.e. fiber optic strand. As demand for transmission capacity further increased, multiple optical signals were transmitted in a single wavelength by time division multiplexing (TDM) the various signals in a known sequence onto a signal carrier wavelength.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher speed access to information. Telecommunications companies have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing optical systems. In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using different optical wavelengths. The pluralities of information carrying wavelengths are combined into a multiple wavelength optical signal, which is transmitted in a single optical fiber. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems were not initially deployed, in part, because of the cost associated with providing electrical signal regeneration equipment for each wavelength throughout the system. However, the development of the erbium doped fiber optical amplifier (EDFA) eliminated the need for electrical amplifiers and the associated costs in many systems, thereby gaining WDM communication systems acceptance in the marketplace.
EDFAs can theoretically be used to amplify signals in an amplification wavelength range spanning from approximately 1500 nm and 1600 nm. However, as shown in FIG. 1, EDFAs do not equally amplify each wavelength in the range. As a result, the relative performance of EDFAs in a transmission system varies depending upon the wavelengths used in the system.
EDFA variations (gain non-uniformities) can be minimized by the judicious selection of the wavelengths and amplifier powers used in a system. For example, WDM systems currently restrict the wavelengths used in the system to between 1540 nm and 1560 nm, which are comparably amplified by EDFAs. As might be expected, restricting system designs to only those wavelengths that are comparably amplified by EDFAs severely limits the number of wavelengths, i.e., channels, that can be used to carry information.
The number of wavelengths in the system can be increased to some extent, if only a small number of amplifiers are used in the system. Wavelengths having differing EDFA amplification characteristics can be used in those systems, because the cumulative effect of highly amplified noise does not swamp out lowly amplified signals when only a few amplifiers are used.
In addition, the wavelength dependence of EDFAs is also a function of the amplification power (gain) of the EDFA, as further shown in FIG. 1. Thus, the amplification power of each EDFA in the system generally must be restricted to minimize amplification variations in the system. The amplifier power limitations, in turn, limit how far apart the EDFAs can be spaced in a system, i.e., the span length.
The WDM system size constraints imposed by EDFA wavelength variations have focused attention on providing amplifier configurations that compensate for the variations and provide more uniform gain for a larger band of wavelengths. Various EDFA configurations have been proposed in the literature to minimize amplifier gain variations. For example, see U.S. Pat. Nos. 5,406,766, 5,541,766, 5,557,442, 5,636,301, and 5,696,615; Sugaya et al., Optical Amplifiers and Their Applications, Technical Digest OSA 1995 v. 18, pp. 158-161/FC3-1; Jacobovitz-Veselka et al., Optical Amplifiers and Their Applications, Technical Digest OSA 1995 v. 18, pp. 162-165/FC3-1; Masuda et al., OSA 1997, pp. 40-3/MC3-1,; Park et al., Electronics Letters, Mar. 5, 1998, Vol. 34, No. 5, Online No. 19980346; and, Dung et al., Electronics Letters, Mar. 19, 1998, v. 34, n. 6, Online No. 19980446.
The above referenced gain flattened EDFA configurations generally attempt to flatten the amplifier gain profile to within an approximately 1 dB range over a range of wavelengths and/or amplification powers as the signal exits the EDFA. The bandwidth of the amplifier is typically defined as the wavelength range over which there is a 3 dB variation in the gain profile. The improved gain flattened amplifier characteristics provide some improvement in the number of amplifiers, amplifier power ranges, and span lengths before the signal must be regenerated.
While an improvement, the gain profile variations of the various amplifier configurations nonetheless limit the number of amplifiers that can be used in a WDM system prior to signal regeneration. In order to increase the number of amplifiers used in these systems, each amplifier must be more tightly controlled to minimize amplifier variations. A system in which amplifier control complexity increases with the number of amplifiers is clearly undesirable from both a system management and a cost standpoint.
Thus, the present systems do not provide an effective method to overcome the inherent gain variation in EDFAs and provide for continued growth and development of communication systems. Accordingly, there is a need for optical system controllers that allow for increased network capacity and flexibility. One aspect of which is to reduce the limitations placed on the system by amplification components and provide for a more flexible, longer distance transmission system.